You didn't state it was Titan Ex in your OP. Also, once you get it targeted, as long as you are close to Titan it doesn't matter if there is a target ring or not. I've done it from both ways. When I tank Titan, I know I have to take a couple steps forward once he places himself in front of me so I can actually hit the heart. No big deal. When I DPS Titan with my monk, same thing. It's not hard to see how he's standing, get close, and still be able to shift sides from flank to back as necessary.